Here is the relevant code: http://edocket.access.gpo.gov/cfr_2009/julqtr/pdf/39cfr232.1.pdf
Note: It covers “all real property” not just buildings.

“(l) Weapons and explosives. Notwithstanding the provisions of any other law, rule or regulation, no person while on postal property may carry firearms, other dangerous or deadly weapons, or explosives, either openly or concealed, or store the same on postal property, except for official purposes.”

Interesting to start out with the phrase ‘notwithstanding any other law’ since statutory law enables the USPS regulations so the regulation by its very nature cannot contravene statutory or constitutional law or prohibitions (on government) such as the 2nd Amendment. However at the very end of the regulation:

“Nothing contained in these rules and regulations shall be construed to abrogate any other Federal laws or regulations of any State and local laws and regulations applicable to any area in which the property is situated.” This seems to indicate a legislative ( can you really call regulations ‘legislative’? ) intent to make the regulation subservient to state & local laws and regulations.

Here is a previous analysis of the USPS situation: http://www.buckeyefirearms.org/Concealed-carry-in-a-post-office-may-lead-to-rude-awakening

Every post office I’ve been in has the ubiquitous and incomplete Title 18 Section 930 warnings in them and as the previous link points out, it looks inapplicable.

The government never admits wrongdoing, and never voluntarily gives up authority or power. It shouldn’t take USPS as long to lose as it did NPS.

An interesting and yet still overlooked aspect of the prohibitions the 2nd Amendment places on the federal, state & local governments is the aspect of what “Arms” are protected besides guns. Knives? What about common “less lethal” implements? Certainly we think that the term “Arms” encompasses far more than just firearms. Interestingly we need not wait for Columbia or any other government to enact such a ban. The National Park Service still maintain and enforce a ban on stun guns & chemical irritants through their odious, self defense hating CFR 36 regulation 2.4. This regulation is ripe for a challenge in the post – Heller world regardless of the Coburn Amendment effectively ending the NPS era of oppressing self defense rights in National Parks.

What are “Arms” and to what does the term extend? We believe that if it is allowed the government, it is covered by the 2nd Amendment, period. That may sound extreme to some, but who really is being extreme? Is it those who wish the government to abide by the founding rules of the republic or those who would prohibit arms to the very people the Constitution is meant to protect?